1. Field of the Invention
The invention relates to apparatus for a Coriolis mass flow meter and, in particular, to apparatus for driving a flow conduit used in such a meter and to apparatus for sensing the velocity of the flow conduit.
2. Description of the Prior Art
Coriolis mass flow meters can be used to measure the mass flow of a process fluid. As disclosed in the art, such as in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et al on Jan. 1, 1985 and hereinafter referred to as the U.S. Pat. No. 4,491,025), a Coriolis mass flow meter contains one or two parallel conduits, each typically being a U-shaped flow conduit. Each flow conduit is driven to oscillate about an axis to create a rotational frame of reference. For a U-shaped flow conduit, this axis can be termed the bending axis. As process fluid flows through each oscillating flow conduit, movement of the fluid produces reactionary Coriolis forces that are perpendicularly oriented to both the velocity of the fluid and the angular velocity of the conduit (tube). These reactionary Coriolis forces cause each conduit to twist about a torsional axis that for a U-shaped flow conduit is normal to its bending axis. The amount of twist is proportional to the mass flow rate of the process fluid that flows through the tube.
Coriolis mass flow meters which would utilize small diameter flow conduits, i.e. 0.065" (1.65 mm) OD and 0.125" (3.17 mm) OD tubes, often use only a single flow conduit. The reason for this is that the combined mass represented by the flow conduit and the fluid moving therethrough is substantially less than the mass of the base that supports the flow conduit. Consequently, the base remains substantially rigid when the flow conduit and fluid oscillate. Such a single tube Coriolis mass flow meter is described in U.S. pat. No. Re 31,450. With larger diameter tubes, starting at about 0.25" (6.35 mm) OD, the combined mass of each tube and the fluid represents a much greater proportion of the total mass of the system and, as a result, with such large diameter tubes it is preferable to use a parallel multi-conduit apparatus such as that described in the U.S. Pat. No. 4,491,025.
As disclosed in the above-referenced U.S. patents, a typical Coriolis mass flow meter uses a drive mechanism to oscillate the flow conduit(s) and magnetic velocity sensors to determine the relative velocity of the side legs of a typical flow conduit. Further, a magnet, which forms part of the drive mechanism, and magnetic velocity sensing pick-off coils are usually mounted directly to the flow conduit in a typical single tube Coriolis mass flow meter. For example, in one embodiment known in the art, a 0.062" diameter by 0.343" (1.57 mm by 8.71 mm) cylindrical ALNICO permanent magnet is used as part of the drive mechanism. In such meters, the combined mass of the drive magnet and the pick-off coils, together with their leads, can represent a substantial portion of the total mass of a flow conduit assembly.
Efforts have been made to reduce this mass. For example, coils are often wound with 50 gauge wire. However, because 50 gauge wire is very fine, i.e. finer than human hair, this wire is easily broken. In addition, care must be taken during fabrication of the coil to ensure that the wire is not overstressed to the point where the wire breaks or causes shorted turns within the coil.
Another problem occurring with tube-mounted coils, other than the increased mass, involves the lead wires connected to the coils.
These lead wires are typically wound around the legs of a flow conduit and are typically fastened to the flow conduit using an adhesive, such as lacquer. As a consequence, damping or driving forces are coupled to the oscillating flow conduit via the lead wires and disadvantageously alter the motion of the flow conduit and thereby cause measurement errors. This error is unacceptable in certain applications. Specifically, damping forces typically arise from two sources: (1) friction arising between wires themselves or, in some arrangements, between the wires and adjacent structure--it has been discovered that these frictional forces arise even where the insulating material on the wires is a synthetic resin polymer lubricating material such as is sold under the trademark TEFLON (TEFLON is a trademark of the Dupont Corporation)--and (2) the internal structure of the wire material itself. Driving forces typically arise from adjacent vibrating machinery.
The above-described problem caused by coupling damping or driving forces to the flow conduit is exacerbated when wires are wrapped or attached to more than one portion of a flow conduit. This occurs because the damping or driving forces that are coupled to the legs of a U-shaped flow conduit are generally unequal. As a result, the tube can twist under the influence of the different forces, which additional twist can mask the twisting action caused by the Coriolis forces.
Attempts to alleviate some of the above-identified problems include wrapping or affixing wires, for example, by glue or tape, to a flow conduit. Because the wires are oriented substantially parallel to the tube to which they are affixed, these wires are substantially prevented from kinking.
An additional problem that occurs with respect to the wires is fatigue. If the mechanical characteristics of the wires are at least equivalent to or better than those of the oscillated structure, mechanical fatigue of the wires presents an engineering problem that is comparable to that for the oscillated structure. However, whenever wires are wrapped, taped or glued to a flow conduit, additional mass is often added to the flow conduit. This additional mass arises from either the additional length of wire required when wires are wound around the flow conduits or from the added tape or glue. This additional mass can alter the oscillatory motion of the conduit. In addition, because the effects of humidity and temperature on glue are not uniform, differential damping induced by the glue can also alter the oscillatory motion of the tube.
As a result, in fabricating Coriolis mass flow meters it would be advantageous to be able to use reduced mass drive mechanisms and velocity sensors, to reduce the coupling of driving and/or damping forces to the flow conduits, to reduce the problems of friction and temperature, and to ease assembly of the pick-off coils by allowing larger gauge wire to be used in their construction.